This study focuses on Provocative Question 1: What molecular mechanisms influence penetrance in individuals who inherit a cancer susceptibility gene? Lynch syndrome is a genetic disease predisposing to colorectal (CRC) and other cancers that affects >1 million Americans. Germline mutations in DNA mismatch repair (MMR) genes MLH1, MSH2, MSH6 and PMS2 cause MMR deficiency (dMMR) and Lynch syndrome. Lynch syndrome CRCs have greatly elevated missense and small in/del frameshift mutation rates. Lynch syndrome has incomplete disease penetrance that varies widely. Here we will mechanistically elucidate two important questions about Lynch syndrome CRC penetrance. First, immune checkpoint blockade studies have revealed important roles for adaptive immunity against tumor mutation associated neoantigens (MANAs) in late stage and metastatic Lynch syndrome /dMMR malignancies. However, in the setting of pre-malignancy and early-stage CRCs, where the MANA burden is much lower, does adaptive immunity suppress Lynch syndrome penetrance? Second, the microbiota Fusobacterium nucleatum (F. nucleatum) is consistently associated with increased CRC risk. For Lynch syndrome CRCs, what are the mechanisms through which F. nucleatum promotes penetrance? In part these questions remain unanswered because Lynch syndrome mouse models develop few CRCs. To elucidate the mechanisms influencing Lynch syndrome CRC penetrance, we have developed the first robust mouse Lynch syndrome CRC model. In Aim 1 we will elucidate the roles of adaptive immunity mechanisms to reduce mouse - Lynch syndrome CRC penetrance. We will test hypotheses that neoantigen vaccination reduces overall mouse Lynch syndrome CRC penetrance, and specifically for CRCs that arise from Lgr5+ cancer stem cells. In Aim 2 we will elucidate mechanisms of F. nucleatum to promote Lynch syndrome CRC penetrance. We will test hypotheses that F. nucleatum promotes IL17A driven Lynch Syndrome Lgr5+ cancer stem cell proliferation, that F. nucleatum mono-association promotes mouse Lynch syndrome CRC penetrance, and that vaccination with F. nucleatum antigens reduces colonization and CRC penetrance in Lynch Syndrome mice. Overall these studies will use state-of-the-art tools to elucidate adaptive immune mechanisms influencing Lynch syndrome CRC penetrance.